7473 Standard 6 Mo Alloys and Advanced 6 Mo Alloy

Standard 6Mo alloys, such as Cronifer 1925hMo, 254SMO, Inco 25-6Mo, and Al-6XN were basically derived from alloy 904L metallurgy by increasing the molybdenum content by approximately 2% and fortifying it with nitrogen as a cost-effective substitute for nickel for metallurgical balance and improved thermal stability. The addition of molybdenum and nitrogen provided the added benefits of improved mechanical properties and resistance to localized corrosion. These alloys are readily weldable with an overalloyed filler metal such as alloy 625, C-276, or 59 to compensate for the segregation of molybdenum in the interdendritic regions of the weld. They have been extensively used in offshore and marine, pulp and paper, flue gas desulfurization (FGD), chemical process industry for both organic and inorganic compounds, and a variety of other applications. The 6Mo family alloys successfully bridge the performance gap between standard stainless steels and the high-performance nickel-based alloys in a cost-effective manner.

A higher chromium/higher nickel version of standard 6Mo alloys is the new advanced 6Mo alloy Nicrofer 3127hMo—alloy 31—UNS N08031. Its greatly improved corrosion resistance compared with conventional 6Mo family alloys and alloy 28 is achieved via increased Cr (27%) and Mo (6.5%) contents and fortification with nitrogen (0.2%). Alloy 31's corrosion behavior, achieved with only about half the nickel content of alloy 625, makes it a very cost-effective alternative in many applications. The pitting potential of this alloy as determined in artificial seawater makes it a suitable alloy for heat exchangers using seawater or brackish water as cooling media. Its corrosion resistance in sulfuric acid in medium concentration ranges is superior even to that of alloy C-276 and alloy 20 (Table 7.4). Its localized corrosion resistance is superior to many alloys including alloy 625. (Table 7.5). Figure 7.1 shows this alloy to maintain its corrosion pitting potential as a function of temperature in artificial seawater. This property has led to its use in tube-and-shell heat exchangers using seawater or brackish water as a cooling medium.

However, in view of the specific active/passive characteristics of alloy 31 in sulfuric acid environments, one must be extremely careful, when specifying this material for sulfuric acid use at 80% concentration and temperatures above 80°C, because these conditions will render the material active. Alloy 31 has been

TABLE 7.4 Corrosion Resistance in Sulfuric Acid [Corrosion Rate (mpy)]
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